Learning Objectives
- Understand the role of meiosis in ensuring genetic variation through the production of non-identical gametes
- Explain how independent assortment of chromosomes in metaphase 1 creates genetic variation
- Describe how crossing over of alleles between chromatids in prophase 1 increases variation
- Compare and contrast the stages of meiosis with those of mitosis
Why Sexual Reproduction Matters
Organisms need to reproduce to pass on their genes to the next generation. But why have most complex organisms evolved to use sexual reproduction when asexual reproduction is simpler and faster?
The answer lies in genetic variation – the key to survival in a changing environment.
🔄 Asexual Reproduction
- Produces genetically identical offspring (clones)
- Only requires one parent
- Fast and energy efficient
- Very limited genetic variation (only from mutations)
- Population vulnerable to environmental changes
- Common in bacteria, some plants, some invertebrates
🧬 Sexual Reproduction
- Produces genetically unique offspring
- Requires two parents (usually)
- Slower and requires more energy
- High genetic variation from multiple mechanisms
- Population can adapt to changing conditions
- Dominant in animals and flowering plants
💭 Think About It
In a changing environment (new diseases, climate change, new predators), which population would be more likely to survive – one with high genetic variation or one where all individuals are identical? Why?
Check Your Understanding
State two advantages of sexual reproduction over asexual reproduction.
Mark Scheme
Alternative acceptable answers: removes harmful mutations from population; increases evolutionary potential
Explain why organisms that only reproduce asexually may be at a disadvantage if a new disease enters their population.
Mark Scheme
What Are Gametes?
Sexual reproduction requires the fusion of two specialised sex cells called gametes. When gametes fuse during fertilisation, they form a zygote – the first cell of a new individual.
For this to work without doubling the chromosome number each generation, gametes must contain half the normal number of chromosomes.
Where Are Gametes Formed?
Gametes are produced in specialised sex organs called gonads:
👨 In Males
- Testes produce sperm (spermatozoa)
- Sperm are small, numerous, and motile
- In plants: anthers produce pollen
👩 In Females
- Ovaries produce ova (eggs)
- Ova are large, few in number, and immotile
- In plants: ovules produce egg cells
Quick Retrieval Practice
Click each card to test yourself, then reveal the answer:
The Stages of Meiosis
Meiosis is a special type of cell division that produces four haploid daughter cells, each genetically different from each other and from the parent cell.
Unlike mitosis (one division), meiosis involves two successive divisions:
- Meiosis I – The reduction division (separates homologous pairs)
- Meiosis II – Similar to mitosis (separates sister chromatids)
- Chromosomes condense and become visible
- Each chromosome consists of two sister chromatids
- Homologous pairs come together (synapsis)
- Crossing over occurs – chromatids exchange genetic material at chiasmata
- Nuclear membrane breaks down
- Spindle fibres form and attach to centromeres
- Homologous pairs line up on the metaphase plate
- Random orientation of each pair – maternal or paternal chromosome can face either pole
- This is independent assortment
- Homologous chromosomes are pulled apart
- One chromosome from each pair moves to each pole
- Centromeres do NOT divide – sister chromatids stay together
- Chromosome number is halved
- Chromosomes reach opposite poles
- Nuclear membranes may re-form
- Cytokinesis produces two haploid cells
- Each cell has one chromosome from each homologous pair
- No DNA replication before meiosis II
- Chromosomes condense (if they decondensed)
- New spindle fibres form at right angles to first spindle
- Nuclear membrane breaks down
- Individual chromosomes line up on the metaphase plate
- Spindle fibres attach to centromeres
- Similar to metaphase in mitosis
- Centromeres divide
- Sister chromatids are pulled to opposite poles
- Sister chromatids are now individual chromosomes
- Chromosomes reach poles and decondense
- Nuclear membranes re-form
- Cytokinesis produces four haploid daughter cells
- Each cell is genetically unique
Meiosis Under the Microscope
This diagram shows the complete process of meiosis from a single diploid cell to four haploid daughter cells:
Check Your Understanding
Compare and contrast the events of anaphase 1 and anaphase 2 in meiosis.
Mark Scheme
Explain why there is no DNA replication between meiosis I and meiosis II.
Mark Scheme
How Meiosis Creates Genetic Variation
Meiosis is crucial for sexual reproduction because it generates genetic variation through two key mechanisms. This variation is why you don't look identical to your siblings (unless you're an identical twin!).
🔀 Crossing Over (Recombination)
During prophase 1, homologous chromosomes pair up closely. At points called chiasmata, the chromatids break and exchange segments of DNA.
What happens: Maternal and paternal chromatids exchange genetic material
This creates recombinant chromosomes that contain a unique mixture of alleles from both parents. The more chiasmata that form, the more genetic variation is produced.
💡 Errors during crossing over can also introduce mutations, adding even more variation.
🎲 Independent Assortment
During metaphase 1, homologous pairs line up randomly on the metaphase plate. Which way each pair faces is completely independent of the others.
What happens: Random orientation of homologous pairs
In humans with 23 pairs of chromosomes, this creates 2²³ = over 8 million possible combinations in the gametes!
💡 Combined with crossing over, the number of unique gametes is essentially infinite.
💭 Mathematical Connection
Calculate the number of possible chromosome combinations from independent assortment alone for an organism with:
- 4 pairs of chromosomes: 2⁴ = 16 combinations
- 10 pairs of chromosomes: 2¹⁰ = 1,024 combinations
- 23 pairs of chromosomes (humans): 2²³ = 8,388,608 combinations
And remember – this is just from independent assortment. Crossing over creates countless more variations!
Check Your Understanding
Describe how crossing over during meiosis leads to genetic variation in gametes.
Mark Scheme
Explain how meiosis produces genetically different gametes through the processes of crossing over and independent assortment.
Mark Scheme – Crossing Over (3 marks)
Mark Scheme – Independent Assortment (3 marks)
Exam Practice & Technique
How to Approach 6-Mark Questions on Meiosis
"Describe" = say what happens. "Explain" = say what happens AND why. "Compare" = identify similarities and differences.
For meiosis questions, think: WHERE does it happen? WHEN in the process? WHAT exactly occurs? WHY does it matter for variation?
Examiners look for key terms: homologous pairs, sister chromatids, chiasmata, centromere, spindle fibres, metaphase plate, haploid, diploid.
Always connect what happens to WHY it matters. Crossing over → new allele combinations. Independent assortment → 2ⁿ combinations.
Worked Example
Describe the events of meiosis I that lead to the production of two haploid cells from one diploid cell.
Model Answer
During prophase 1, chromosomes condense and become visible. [Stage identified ✓] Homologous chromosomes pair up in a process called synapsis, forming bivalents. [Key terminology ✓] Crossing over may occur at points called chiasmata, where non-sister chromatids exchange genetic material. [Process described ✓]
In metaphase 1, the homologous pairs line up on the metaphase plate. [Stage identified ✓] Spindle fibres attach to the centromeres of each chromosome. The orientation of each pair is random, leading to independent assortment. [Key concept ✓]
During anaphase 1, the homologous chromosomes are pulled to opposite poles by the spindle fibres. [Process described ✓] Importantly, the centromeres do not divide, so sister chromatids remain together. This halves the chromosome number.
In telophase 1, the chromosomes reach the poles and nuclear membranes may reform. [Stage identified ✓] Cytokinesis divides the cytoplasm, producing two haploid cells, each containing one chromosome from each homologous pair. [Outcome stated ✓]
Practice Question
A student states that "meiosis is just mitosis happening twice." Evaluate this statement, explaining the key differences between meiosis and mitosis.
Mark Scheme
Award max 5 marks
Key Vocabulary
Make sure you understand and can use these terms correctly:
Final Review - Test Yourself!
Cover the answers and see how many you can recall: